Positive displacement compressors are normally operated over a range of capacities and thus require some means for modifying their operation if efficient operation is to be maintained. It is desirable to be able to unload a compressor to various percentages of capacity in fixed increments or over an entire range. Simultaneously, it is desirable to efficiently maintain the desired discharge pressure to suction pressure ratio, or V.sub.i, for meeting system requirements. To meet these various requirements, a number of individual controls are used. In screw compressors, for example, capacity control is conventionally achieved by the use of a slide valve. The slide valve is located in and reciprocates in the cusp of the housing formed between the intersecting bores for the two rotors. The slide valve thus defines a portion of each bore and thereby compromises the integrity of the housing as well as making for a complicated device. The slide valve is reciprocatably positionable with respect to the axes of the rotors and can thus effectively change the start of compression by changing the closing point of the suction stroke and thereby controlling the amount of gas trapped and compressed.